Coal-fired utility boilers and other combustion systems, such as municipal solid waste incinerators, produce emissions which contain oxides of nitrogen (NOx) and oxides of sulfur (SO.sub.2), both of which have been found to cause acid rain. Studies have demonstrated that the NOx-related portion of acid rain can produce extensive damage to trees and forests. Thus, it is important to reduce NOx emissions from such combustion systems to the extent possible. While it is desirable to provide for reduction of NOx emissions from combustion systems, there is currently no demonstrated non-catalytic technology capable of removing in excess of 60% to 70% of the NOx.
One presently known catalytic system for removing NOx, called the selective catalytic reduction (SCR) system, provides for injection of ammonia gas into a NOx-containing flue gas to thereby react the ammonia with NOx over a catalyst at temperatures of about 700.degree. F. to produce nitrogen gas and water vapor as by-products. Typical NOx reduction levels are 80%, using the SCR system, with the reported cost varying between about $60/kw to $120/kw, depending on site conditions. The operating cost of SCR technology is high, due in part to the high cost of catalyst replacement which is required about once every two years. SCR technology is not considered applicable to incinerators due to the contamination and poisoning of the catalyst.
Another presently known NOx control system, called the selective non-catalytic reduction (SNCR) system, provides for injection of gaseous ammonia or liquid phase urea into a flue gas at temperatures above 1400.degree. F. to reduce NO to nitrogen. The SNCR systems suffer the disadvantage that, if sufficient ammonia or urea is injected to achieve high NOx removal efficiency, there may be an unacceptably high degree of ammonia slippage. (As used herein, "ammonia slippage" or "ammonia slip" means to concentration of ammonia gas contained in the flue gas exit from the NOx control process.) The ammonia slip combines with SO.sub.2, SO.sub.3, HCl and HF to form ammonia salts at temperatures typically less than 500.degree. F. When such salts condense, solid particulate is formed, which may cause deposits in critical zones, such as the air preheater systems in conventional boilers. In order to prevent this problem, less ammonia or urea is injected, and the overall NOx reduction capability of SNCR systems is generally limited to between 30% and 60%. This level of performance is unacceptably low.
In order to overcome the problems associated with the SCR and SNCR systems, I conceived and developed several alternative NOx reduction processes and the systems useful for carrying them out. A first such process is set forth in U.S. Pat. No. 4,783,325 issued Nov. 8, 1988 entitled "Process and Apparatus for Removing Oxides of Nitrogen and Sulfur from Combustion Gases." U.S. Pat. No. 4,783,325 is incorporated herein by this reference. The process disclosed in my '393 application preferably operates between 800.degree. F. and 1400.degree. F., and utilizes a peroxyl initiator (an injection chemical), such as methanol, dispersed or mixed in a carrier gas which is injected into a flue gas to contact and convert NO contained in the Flue gas to NO.sub.2. NO.sub.2 is then removed from the flue gas prior to its discharge into the atmosphere by means of a dry particulate absorbent and/or by wet scrubbing, or the like. (A "peroxyl initiator," as used herein, is a material which reacts with oxygen (O.sub.2) to form the peroxyl (HO.sub.2) radical. Such materials include hydrocarbons, such as propane, methane, and the like, oxygen substituted hydrocarbons, such as methanol and ethanol, as well as hydrogen, and hydrogen peroxide.)
A second such alternative process that I conceived and developed is set forth in a patent application that I filed via the Patent Cooperation Treaty on Feb. 2, 1988, assigned International Application No. PCT/US88/ 00463, and titled "Process and Apparatus Using Two-Stage Boiler Injection for Reduction of Oxides of Nitrogen." The U.S. Pat. No. 4,783,325, which is incorporated herein, discloses a process where an NHi precursor material (an injection chemical) is mixed in a carrier gas and injected into a flue gas stream at a temperature greater than 1400.degree. F. in a first injection zone to reduce NO to nitrogen, followed by mixing a peroxyl initiator in a carrier gas and injecting this second mixture into the flue gas in a second injection zone downstream from the first injection zone at a temperature less than 1400.degree. F. to oxidize residual NO to NO.sub.2. The NO.sub.2 may then be removed from the flue gas by conventional means prior to its discharge into the atmosphere. ("NHi precursors," as used herein, are materials, such as ammonia, urea, cyanuric acid, biurst, triuret, ammelide, or mixtures thereof, which react to form NHi radicals, i.e., short-lived molecules comprising one nitrogen atom and one or more hydrogen atoms, such as NH, NH.sub.2 and NH.sub.3.)
I discovered, however, that, because the chemical reactions between the injection chemicals and the NOx molecules in the flue gas take place so rapidly at elevated temperature, the presently available injection systems, including available mixing nozzles, do not provide for the desired amount of cross-sectional coverage and mixing of the injection chemical in large flue gas ducts in a short enough time to provide for desirably higher levels of NOx removal. For example, presently available nozzle systems have the object of providing highly atomized liquid droplets while using a minimum amount of atomizing media, such as compressed air. Such a highly atomized cloud of droplets is difficult to inject with sufficient velocity to penetrate across a large flue gas duct. It is also important that such penetration be made as rapidly as possible so that the injection chemicals contact and react with the maximum amount of flue gas to thereby convert NO to N2 (as is the case with the NHi precursor), or to convert NO to NO.sub.2, as in the case with the peroxyl initiator.
Thus, techniques and associated systems are needed that will provide for, first, adequately dispersing injection chemicals in a carrier gas and, second, for injection the resulting mixture at a sufficiently high velocity into the flue gas for maximizing rapid contact between the injection chemicals and the NOx molecules in the flue gas.
There is also a need for techniques and associated systems that are effective for controlling the temperature of flue gas. For example, if an excess of peroxyl initiator material is injected, the resulting exothermic reactions can increase flue gas temperature. This technique is described as the "reburning" or "two-stage combustion" process for reducing NO to nitrogen. However, in the context of the present invention, means are provided for injecting the second stage fuel, i.e., the peroxyl initiator, at high velocity and in a unique manner, so that the flue gas temperature can be controlled. If, in an incineration system, for example, it is desired to control the temperature of the flue gas at 1800.degree. F., i.e., greater than 1400.degree. F., then the peroxyl radical will not be formed in large quantities, and NO will be preferentially reduced to N.sub.2 by the "reburning" process since the peroxyl initiators defined above would all be considered "clean" fuels containing low levels of fuel nitrogen content.